In a recent trend of high integration of semiconductor devices, design rule is becoming stricter. Accordingly, a lithography process performed during a manufacturing process requires a high degree of accuracy.
For this reason, a resist used in the lithography process includes a resist layer subject to a wet developing process (hereinafter, referred to as “lower layer resist”) provided beneath a wet-developed resist layer (hereinafter, referred to as “upper layer resist”), for example. Such resist structure enables a high density patterning, despite, if any, a stepped portion in an etching layer therebelow.
However, in order to maintain the accuracy when the lower layer resist of the double-layer structure resist is dry-developed, the lower layer resist needs to be sufficiently etched without destroying the upper layer resist by etching. Additionally, it is crucial that sidewalls of a pattern formed in the lower layer resist are vertically formed in a designed dimensions in order to improve an accuracy of machining.
FIG. 4 is a schematic crosssectional view of a processing object 10 developed by using a conventional method. FIG. 4A shows a case where a reactive ion etching is performed by using, e.g., an oxygen gas, whereas FIG. 4B illustrates a case where an etching is carried out by using a gaseous mixture of, e.g., a nitrogen gas and a hydrogen gas. Herein, the processing object 10 includes an upper layer resist 16 forming a top surface thereof, a lower layer resist 14 formed under the upper layer resist 16 and an etching layer 12 formed beneath the lower layer resist 14.
Since, however, the etching by using the oxygen gas is isotropically performed with minimal anisotropy, and thus, an erosion phenomenon, e.g., an undercut or a bowing, occurs at the sidewall of the pattern in the lower layer resist 14, as illustrated in FIG. 4A.
To that end, there is provided a method of low power etching to avoid the erosion phenomenon described above. However, such low power etching method suffers from drawbacks of reduced etching rateand deteriorated efficiency.
On the other hand, by using the gaseous mixture of nitrogen gas and hydrogen gas in etching, the undercutting of the sidewall or the like can be avoided. Since, however, an etching selectivity between the lower layer resist 14 and the upper layer resist 16 is small, when using such gaseous mixture in performing etching, a pattern formed in the upper layer resist 16 is damaged, thereby deteriorating the machining accuracy of the lower layer resist 14 as well as the etching accuracy of the etching layer 12 formed beneath the lower layer resist 14.